Aqueous-based method for producing ultra-fine metal powders

ABSTRACT

The present invention provides a method for forming compositions having a plurality of ultra-fine metallic particles, and the metallic composition produced therewith. Also provided is a substrate coated with the plurality of ultra-fine metallic particles obtained in accordance with the method of the present invention.

FIELD OF THE INVENTION

The present invention relates generally to ultra-fine metalliccompositions and methods of making thereof. The present inventionfurther relates to methods of coating various substrates with theultra-fine metallic compositions.

BACKGROUND OF THE INVENTION

Ultra-fine metallic particles have many unique physical and chemicalcharacteristics, which make them ideal materials for a variety ofapplications, such as electronics, catalysis, metallurgy, anddecorations. Compared to the various particle-producing techniques usedin the art, the methods based on the chemical precipitation in solutionsprovide several advantages, e.g., low manufacturing cost and a very goodcontrol of the mechanism of metal particles formation. Others in the arthave successfully prepared micron and submicron-size metallic powders ofCo, Cu, Ni, Pb, and Ag using chemical-based techniques, such as the onesbased on the reduction in alcohols or polyols. For example, U.S. Pat.No. 4,539,041 discusses a method for producing micrometer-size metallicparticles by using polyols to convert various metallic compounds intometal powders. Nonetheless, these processes require complex equipmentand the metallic powders produced are generally more expensive becauseof the cost of the organic solvents used. The present invention providesa process capable of generating cost effectively highly dispersedcrystalline ultra-fine metallic particles in aqueous medium, which arehighly desirable in many practical applications, especially inelectronics.

SUMMARY OF THE INVENTION

The present invention provides a method for forming compositions havinga plurality of ultra-fine metallic particles, and the metalliccomposition produced therewith, where the plurality of ultra-finemetallic particles is obtained in accordance with a process including:

-   -   (a) obtaining a reducing solution comprising a reducing agent        and a stabilizing agent;    -   (b) obtaining a metal-ammonia solution containing a        metal-ammonia complex;    -   (c) forming a reaction mixture containing the reducing solution        and the metal-ammonia solution;    -   (d) maintaining the reaction mixture under a suitable condition        for a time effective to reduce the metal-ammonia complex to        metallic particles; and optionally,    -   (e) isolating the metallic particles.        In one embodiment of the present invention, the metal-ammonia        complex is the complex of ammonia with a transition metal or a        noble metal, e.g., Cu, Pd, and Ag, formed by reacting a solution        comprising a metal salt with ammonium hydroxide or ammonia. In        another embodiment, the reducing agent is a saccharide, such as        D-glucose. In yet another embodiment, the stabilizing agent is a        water-soluble resin (e.g., a natural occurring, synthetic, or        semi-synthetic water-soluble resin) or gum arabic. The gum        arabic may be removed during the isolation of the metallic        particles through hydrolysis. The plurality of ultra-fine        metallic particles may have at least one desirable feature, such        as tight size distribution, low degree of agglomeration, high        degree of crystallinity, ability to re-disperse fully into a        liquid (e.g. an aqueous solution) to form stable dispersions.

In another aspect, the present invention provides a substrate coatedwith the plurality of ultra-fine metallic particles obtained inaccordance with the method disclosed herein.

Additional aspects of the present invention will be apparent in view ofthe description that follows.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts an experimental set-up used in the synthesis ofultra-fine silver particles.

FIG. 2 shows the FE-SEM images of ultra-fine silver particles producedusing the method of the present invention. (a) 198.7 g AgNO₃ and flowrate at 8 ml/min; (b) 382 g AgNO₃ and flow rate at 8 ml/min; and (c) 382g AgNO₃ and flow rate at 30 ml/min. Images were acquired using a FE-SEMat two magnifications (25,000 and 100,000).

FIG. 3 illustrates the particle size distribution (PSD) of silverparticles as number (%) (a) and volume (%) (b), obtained from 382 gAgNO₃ at a flow rate of the metallic precursor solution of 30 ml/min.

FIG. 4 shows the X-ray diffraction patterns of silver particles shown inFIG. 2 a.

DETAILED DESCRIPTION OF THE INVENTION

As used herein and in the appended claims, the singular forms “a,” “an,”and “the” include plural references unless the content clearly dictatesotherwise. Thus, for example, reference to “a particle” includes aplurality of such particles and equivalents thereof known to thoseskilled in the art, and reference to “the reducing agent” is a referenceto one or more reducing agent and equivalents thereof known to thoseskilled in the art, and so forth. All publications, patent applications,patents, and other references mentioned herein are incorporated byreference in their entirety.

The present invention generally provides a simple and more costeffective chemical based method for producing highly dispersedultra-fine metallic powders than those known in the art. The presentinvention also provides ultra-fine metallic particles having at leastone desirable feature, such as tight size distribution, low degree ofagglomeration, high degree of crystallinity, ability to re-dispersefully into a liquid (e.g. an aqueous solution) to form stabledispersions.

In at least one embodiment of the invention, the present inventionprovides a method or system beneficially produces metallic powders, andalso metallic powders produced therewith, that include a plurality ofultra-fine metallic particles obtained by (a) obtaining a reducingsolution containing a reducing agent and a stabilizing agent; (b)obtaining an aqueous solution containing a metal-ammonia complex; (c)forming a reaction mixture containing the reducing solution and theaqueous solution of step (b); (d) maintaining the reaction mixture undera suitable condition (e.g. pH) for a time effective to reduce themetal-ammonia complex to metallic particles; and optionally, (e)isolating the metallic particles.

The process of the present invention may be used to manufactureultra-fine particles of various metals, such as Ag, Au, Co, Cr, Cu, Fe,Ir, Mo, Ni, Nb, Os, Pd, Pt, Re, Rh, Ru, Sn, Ta, Ti, V, and W, and alloysor composites containing these metals. The metal-ammonia complex may bemixed with a reducing composition or agent, which converts the metalions to ultra-fine metal particles under various reaction conditions.

The metal-ammonia complex used in the process of the present inventionmay be the complex of ammonium with a variety of metals, including,without limitation, transitional metals and noble metals, such as, Ag,Au, Co, Cr, Cu, Fe, In, Ir, Mn, Mo, Ni, Nb, Os, Pd, Pt, Re, Rh, Ru, Sn,Ta, Ti, V, W, Zn, and the combinations thereof. In one embodiment, themetal-ammonia complex may be obtained by reacting a solution containinga metal salt with ammonium hydroxide or ammonia. For example, 198.7 gAgNO₃ is dissolved in 234 ml DI water in a 2 L glass beaker. After thesilver nitrate is completely dissolved, 195 ml ammonium hydroxide isadded into the silver nitrate solution under stirring. 291 ml DI wateris then added and the overall volume of the silver ammonia solution is720 ml. This solution may be covered with plastic wrap (to avoid theammonia evaporation) and protected with aluminum foil (to prevent theexposure to light).

The term “reducing composition” or “reducing agent,” as used herein andin the appended claims, generally includes any reducing substance, and acombination thereof, which is capable of reducing metal ions to metallicparticles, such as, without limitation, alcohol, aldehydes, aldose,monohydroxylic alcohols, polyhydroxylic alcohols (polyols), hydrazinehydrate, and reducing saccharides (including, e.g., monosaccharides,oligosaccharides, and polysaccarides). Examples of reducing saccharidesinclude, but not limited to, glyceraldehydes, erythrose, threose,ribose, arabinose, xylose, lyxose, allose, altrose, glucose, dextrose,mannose, gulose, idose, galactose, talose, lactose, maltose, isomaltose,cellobiose, and starch. The nature of the reducing species and theircomposition in the process of the present invention may be commanded bythe particular reaction/metallic element.

The term “stabilizing composition” or “stabilizing agent,” as usedherein and in the appended claims, generally includes any stabilizingsubstance, such as, without limitation, water soluble resins (including,e.g., naturally occurring, synthetic, and semi-synthetic water solubleresins), gum arabic, polymers, polysaccharides, glycoproteins, nucleicacids, various salts of naphthalene sulphonic-formaldehyde co-polymers,and a combination thereof, which is capable of dispersing andstabilizing the newly formed ultra-fine metallic particles in thereaction mixture and thus preventing undesirable aggregation of theseparticles such that the size of the resulting metallic particles is lessthan about 10 μm, preferably, less than about 1 μm, and more preferably,less than about 100 nm. As used herein and in the appended claims, theterm “ultra-fine particles” generally includes particles havingdiameters of less than about 10 μm, preferably, less than about 1,000nm, and more preferably, less than about 500 nm, and even morepreferably, less than about 100 nm. The ultra-fine metallic particlesmay be the metallic particles of various metals, including, withoutlimitation, transitional metals and noble metals, such as, Ag, Au, Co,Cr, Cu, Fe, In, Ir, Mn, Mo, Ni, Nb, Os, Pd, Pt, Re, Rh, Ru, Sn, Ta, Ti,V, W, Zn, and the combinations thereof.

The stabilizing composition used in the process of the present inventionmay be commanded by the particular reaction. Examples of suitablestabilizing agents include, without limitation, gum arabic, cellulosederivatives (e.g., carboxymethyl cellulose, carboxyethyl cellulose,methyl cellulose, etc.) and modified products thereof, polyvinyl alcoholand derivatives thereof, polyvinyl pyrrolidone, polyacrylamide andcopolymers thereof; acrylic acid copolymers, vinylmethyl ether-maleicanhydride copolymers, vinyl acetate-maleic anhydride copolymers, varioussalts of naphthalene sulphonic-formaldehyde co-polymers, styrene-maleicanhydride copolymers, calcined dextrin, acid-decomposed dextrin,acid-decomposed etherified dextrin, agarose, and salmon sperm DNA. Inone embodiment of the present invention, the stabilizing agent may begum arabic. In another embodiment of the present invention, thestabilizing agent is a salt of naphthalene sulphonic-formaldehydeco-polymer.

The stabilizing agent, such as gum arabic, may be removed after thereaction. A number of protocols for removing the stabilizing agent areknown in the art, such as, acid, alkaline, and/or enzymatic hydrolysis.In one embodiment, gum arabic may be removed from the reaction mixtureafter the reaction through alkaline hydrolysis. For example, thehydrolysis may be performed for extended time at high temperature (e.g.between 70° and 100° C., or between 80° and 90° C., or between 82° and88° C.) and high pH (e.g. pH 11.5). It is generally desirable tomaintain the pH of the mixture during the hydrolysis at between 9 and14, or between 10 and 12, or between 10.5 and 11.5. The duration of thehydrolysis may be commanded by a number of facts, such as, the amount ofstabilizing agent (e.g. gum arabic) used. In one embodiment, thehydrolysis of the gum may generally be performed for about 0.2 to 10hours, or about 1 to 5 hours, or about 2 to 3 hours.

The resulting ultra-fine metal particles may be obtained followingstandard protocols known in the art, such as by precipitation,filtration, and centrifugation. The particles may further be washed,such as by using methanol or ethanol, and dried, such as by air, N₂, orvacuum.

The ultra-fine metallic particles may also have at least one desirablefeature, such as, tight size distribution, low degree of agglomeration,high degree of crystallinity, ability to re-disperse fully into a liquid(e.g. an aqueous solution) to form stable dispersion, or a combinationthereof.

Unlike other metallic powders appearing in the art, in one embodiment,the system of the present invention produces metallic powders thatinclude ultra-fine metallic particles, particularly, isometricultra-fine metallic particles, that have a tight size distribution. Thebreadth of the size distribution, as used herein, generally refers tothe degree of variation in the diameter of the ultra-fine metallicparticles in a metallic composition. Tight, used in this context,indicates a relatively small variation in the size of the ultra-fineparticles. In one embodiment, the ultra-fine metallic particles aredeemed to have a tight size distribution when the diameters of at leastabout 80%, preferably, at least about 85%, and more preferably, at leastabout 95%, of the ultra-fine metallic particles of the present inventionare within the range of N±15% N, where N is the average diameters of theultra-fine metallic particles. The diameters of the ultra-fine metallicparticles may be measured by a number of techniques, such as, by anelectron microscope, particularly, a scanning electron microscope (e.g.field emission scanning electron microscope).

The metallic powders produced with the system of the present inventionmay also include ultra-fine metallic particles that have a low degree ofagglomeration, as illustrated in FIG. 3. The degree of agglomeration maybe expressed using the index of agglomeration I_(agg1), which is theratio between the average size distribution of the ultra-fine metallicparticles (“PSD50%”) and the average diameter of the particles. Theaverage particle size distribution may be determined by any methodsknown in the art, including, but not limited to, dynamic lightscattering (DLS), laser diffraction, and sedimentation methods, whilethe average particle size may be determined by averaging the diameter ofthe individual ultra-fine metallic particles obtained by, e.g., electronmicroscopy. An I_(agg1) value of 1.0 indicates completely lack ofagglomeration, while an increase in I_(agg1) value indicates an increasein the degree of aggregation. In one embodiment, the powders ofultra-fine metallic particles of the present invention have an I_(agg1)value of 1.2 or less.

The metallic powders produced in accordance with the present inventionmay also include ultra-fine metallic particles that have a high degreeof crystallinity. The term “degree of crystallinity,” as used herein andin the appended claims, generally refers to the ratio between the sizeof the crystallites in the metallic powder and the diameter of themetallic particles. The size of the constituent crystallites may bededuced from XRD measurements using the Sherrer's equation, while theparticle size may be determined by electron microscopy. A larger ratioof the size of the crystallites in comparison to the diameter of themetallic particles indicates an increased degree of crystallinity and alower internal grain boundary surface. In one embodiment, the ultra-finemetallic particles have a high degree of crystallinity if at least about80%, preferably, at least about 85%, more preferably, at least about90-95%, and even more preferably, about 100% of the ultra-fine metallicparticles of the present invention are highly crystalline. The highdegree of crystallinity is reflected by the visible splitting of thepeaks corresponding to the (220), (311), and (222) reflections in theXRD spectrum (see FIG. 4).

The ultra-fine metallic particles produced in accordance with thepresent invention may form a free flowing dry powder in which themajority of the individual particles may not be strongly attached toeach other and may be readily re-dispersed in a liquid of choice.

In another embodiment of the present invention, the ultra-fine metallicparticles forms stable dispersion when re-dispersed into a liquid, suchas water, or an aqueous solution, where the majority of the individualparticles may move substantially freely in the liquid in which they aredispersed. In one embodiment, the particle dispersion is stable for atleast one week. In another embodiment, the particle dispersion is stablefor 12 weeks.

The present invention further provides a substrate coated with aplurality of ultra-fine metallic particles, where the plurality ofultra-fine metallic particles have at least one desired feature, suchas, tight size distribution, a low degree of agglomeration, a highdegree of crystallinity, and oxidation resistance. The term “substrate”as used herein includes, without limitation, metallic subjects (e.g.,metallic particles, flakes, tubes, and sheets), plastic materials,ceramic subjects, fibers, films, glasses, polymers, organic materials(e.g. resins), inorganic materials (e.g., carbon nanotubes), and anyother object capable of being coated with the ultra-fine metallicparticles produced in accordance with the present invention. Theultra-fine metallic particles may be the metallic particles of variousmetals, preferably, Cu, Pd, and Ag.

EXAMPLES

The following examples illustrate the present invention, which are setforth to aid in the understanding of the invention, and should not beconstrued to limit in any way the scope of the invention as defined inthe claims which follow thereafter.

The ultra-fine silver, palladium and copper particles were prepared byreducing the metallic ammonium complex with D-glucose in the presence ofgum arabic. The experimental set-up for these experiments is illustratedin FIG. 1.

Example 1 Materials

Silver nitrate (AgNO₃) was obtained from Ames Goldsmith Corp. (GlensFalls, N.Y.). Gum arabic was obtained from Frutarom Incorporated (NorthBergen, N.J.). Ammonium hydroxide (NH₄OH) was purchased from FischerScientific Co. (Fair Lawn, N.J.). Acetone, ethanol, and sodium hydroxide(NaOH) solution (10 N) were supplied by Alfa Aesar (Ward Hill, Mass.).D-glucose was purchased from Avocado Research Chemicals Ltd. (ShoreRoad, Heyshane, Lancs.). Cupric nitrate hydrate [Cu(NO₃)₂ 2 1/2H₂O] wasobtained from T.J. Baker Chemical Co. (Phillipsburg, N.J.), while thepalladium nitrate solution 9.0% was obtained from Umicore (SouthPlainfield, N.J.).

Example 2 Preparation of Ultra-Fine Silver Particles (A) Preparation ofthe Reducing Solution

3 L deionised (“DI”) water was heated to 55° C. in a 8 L stainless steelbeaker. When the temperature reaches 55° C., 62.5 g gum arabic wasslowly added into the water and dissolved by stirring the solution witha stirring propeller at low speed for 55 minutes. 36 g of D-glucose werethen added to the solution. The mixture was stirred at 1700 rpm for 5minutes.

(B) Preparation of Silver Ammonium Complex Solution

198.7 g AgNO₃ were dissolved in 234 ml DI water in a 2 L glass beaker.After the silver nitrate was completely dissolved, 195 ml ammoniumhydroxide was added under stirring, followed by the addition of 291 mlDI water to reach a final volume of 720 ml.

(C) Preparation of Ultra-Fine Silver Particles

The reduction process was conducted by pumping the silver ammoniumsolution into the reducing solution at a flow rate of 8 ml/min using aperistaltic pump. When the addition of the silver complex solution iscompleted, the temperature was brought to 80° C. The process wasconducted under continued stirring (1700 rpm).

(D) Hydrolysis of Gum Arabic

The excess of gum arabic was removed by increasing the pH of thedispersion to 11.5 with 10.0 N sodium hydroxide at the temperature ofabout 85° C. The dispersion was maintained in the condition for 2.5hours.

(D) Processing the Silver Powder

When the hydrolysis of the gum was complete, the dispersion was allowedto cool and the silver particles to settle. The supernatant was thendiscarded and the silver particles were washed with water through 3successive decantations. During the last wash, 50% ethanol (in DI water)was added to the settled metallic deposit instead of DI water. Two morewashes with pure alcohol were performed. The powder was then driedovernight on filter paper at room temperature.

Example 3 Preparation of Ultra-Fine Palladium Particles (A) Preparationof the Reducing Solution

A volume of 500 ml DI water was heated to 70° C. in 2 L glass beaker.When the temperature reaches 70° C., 10 g gum arabic was slowly addedinto the water and dissolved by stirring the solution. 100 g ofD-glucose were then added to the solution and the mixture was stirred at1700 rpm for 5 minutes. The pH of solution was adjusted at 10.5 with10.0 N NaOH.

(B) Preparation of Palladium Ammonium Complex Solution

80 ml ammonium hydroxide was added quickly under stirring to 50 mlPd(NO₃)₂ solution 9.0% in a 0.2 L glass beaker, followed by the additionof 50 ml DI water (final volume: 180 ml).

(C) Preparation of Ultra-Fine Palladium Particles

The reducing reaction was conducted by pumping the palladium ammoniumsolution to the reducing solution at a flow rate of 5 ml/min using aperistaltic pump. When the addition of the palladium complex solution iscompleted, the temperature was brought to 80° C. The process wasconducted under continued stirring (1700 rpm).

The hydrolysis of gum arabic and the processing of palladium powder werecarried out in a similar manner as in Example 2 (steps D and E).

Example 4 Preparation of Ultra-Fine Copper Particles (A) Preparation ofthe Reducing Solution

A volume of 500 ml DI water was heated to 70° C. in 2 L glass beaker.When the temperature reaches 70° C., 25 g gum Arabic was slowly addedinto the water and dissolved by stirring the solution with a stirringpropeller at 1700 rpm for 55 minutes. 100 g of D-glucose were then addedto the solution and the mixture was stirred at 1700 rpm for 5 minutes.The pH of solution was adjusted at 10.5 with 10.0 N NaOH.

(B) Preparation of Copper Ammonium Complex Solution

18.2 g [Cu(NO₃)₂ 2 1/2H₂O] were dissolved in 50 ml DI water in a 0.2 Lglass beaker. After the cupric nitrate was completely dissolved, 100 mlammonium hydroxide was added quickly under stirring, followed by theaddition of 50 ml DI water (overall volume: 200 ml).

(C) Preparation of Ultra-Fine Copper Particles

The reduction process was conducted by pumping the cupric ammoniumsolution to the reducing solution at a flow rate of 5 ml/min using aperistaltic pump. When the addition of the cupric complex solution iscompleted, the temperature was brought to 80° C. The process wasconducted under continued stirring (1700 rpm). The remaining steps aresimilar to those of Example 2.

Discussed below are results obtained by the inventors in connection withthe experiments of Example 1-4:

The size of the silver particles obtained did not undergo a substantialchange when the process was scaled up by a factor of two or when theflow rate of the silver ammonium complex solution was raised to 30ml/min from 8 ml/min, suggesting a minor impact of both parameters onthe size of the particles formed. For all experiments the processingyield was >97%.

The experimental conditions and results of Example 2 are summarized inTable I. TABLE I Reducing solution Silver ammonium solution Bach sizeWater Gum Glucose Water AgNO₃ NH₄OH Flow rate Average Exp. Ag (g) (L)arabic (g) (g) (ml) (g) (ml) (ml/min) size (nm) 1 125 3 62.5 36 525198.7 195 8 ˜70 2 245 4 120 69 775 382 390 8 ˜65 3 245 4 120 69 775 382390 30 ˜65

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be appreciated by oneskilled in the art, from a reading of the disclosure, that variouschanges in form and detail can be made without departing from the truescope of the invention in the appended claims.

1. A metallic composition comprising a plurality of ultra-fine metallicparticles obtained in accordance with a process comprising: (a)obtaining a reducing solution comprising a reducing agent and astabilizing agent; (b) obtaining a metal-ammonia solution comprising ametal-ammonia complex; (c) forming a reaction mixture comprising thereducing solution and the metal-ammonia solution; (d) maintaining thereaction mixture under a suitable condition for a time sufficient toreduce the metal-ammonia complex to metallic particles; and optionally,(e) isolating the metallic particles.
 2. The metallic composition ofclaim 1, wherein the metal-ammonia complex is the complex of ammoniawith one of a transition metal and a noble metal.
 3. The metalliccomposition of claim 1, wherein the metal-ammonia complex is the complexof ammonia with a metal selected from the group consisting of Cu, Pd,and Ag.
 4. The metallic composition of claim 1, wherein themetal-ammonia complex is obtained by reacting a solution comprising ametal salt with one of ammonium hydroxide and ammonia.
 5. The metalliccomposition of claim 1, wherein the reducing agent is a saccharide. 6.The metallic composition of claim 5, wherein the saccharide is analdose.
 7. The metallic composition of claim 6, wherein the aldose isglucose.
 8. The metallic composition of claim 1, wherein the stabilizingagent is a water-soluble resin.
 9. The metallic composition of claim 8,wherein the water-soluble resin is a naturally occurring water-solubleresin.
 10. The metallic composition of claim 1, wherein the stabilizingagent is one of a gum arabic and a salt of naphthalenesulphonic-formaldehyde co-polymers.
 11. The metallic composition ofclaim 1, wherein the stabilizing agent is removed during the isolationof the metallic particles.
 12. The metallic composition of claim 11,wherein the stabilizing agent is removed through hydrolysis.
 13. Themetallic composition of claim 1, wherein the plurality of ultra-finemetallic particles has an average size of less than about 100 nm. 14.The metallic composition of claim 1, wherein the plurality of ultra-finemetallic particles has a tight size distribution.
 15. The metalliccomposition of claim 14, wherein the plurality of ultra-fine metallicparticles have a tight size distribution when at least about 80% of theplurality of ultra-fine metallic particles has a diameter within a rangeof N±15% N, wherein N is the average diameter of the plurality ofultra-fine metallic particles.
 16. The metallic composition of claim 1,wherein the plurality of ultra-fine metallic particles has a high degreeof crystallinity.
 17. The metallic composition of claim 16, wherein atleast about 80% of the plurality of ultra-fine metallic particles ishighly crystalline.
 18. The metallic composition of claim 16, whereinabout 100% of the plurality of ultra-fine metallic particles is highlycrystalline.
 19. The metallic composition of claim 1, wherein theplurality of ultra-fine metallic particles has a low degree ofagglomeration.
 20. The metallic composition of claim 19, wherein thedegree of agglomeration is measured with an I_(agg) value and whereinthe I_(agg1) of the plurality of ultra-fine metallic particles is lessthan about 1.2.
 21. The metallic composition of claim 1, wherein atleast about 80% of the plurality of ultra-fine metallic particles is notirreversibly aggregated.
 22. The metallic composition of claim 1,wherein the plurality of ultra-fine metallic particles when re-dispersedinto a liquid forms dispersion which is stable for at least one week.23. The metallic composition of claim 22, wherein the liquid is water.24. The metallic composition of claim 23, wherein the dispersion isstable for 12 weeks.
 25. A method for forming a plurality of ultra-finemetallic particles comprising: (a) obtaining a reducing solutioncomprising a reducing agent and a stabilizing agent; (b) obtaining ametal-ammonia solution comprising a metal-ammonia complex; (c) forming areaction mixture comprising the reducing solution and the metal-ammoniasolution; (d) maintaining the reaction mixture under a suitablecondition for a time effective to reduce the metal-ammonia complex tometallic particles; and optionally, (e) isolating the metallicparticles.
 26. The method of claim 25, wherein the metal-ammonia complexis the complex of ammonia with one of a transition metal and a noblemetal.
 27. The method of claim 25, wherein the metal-ammonia complex isthe complex of ammonia with a metal selected from the group consistingof Cu, Pd, and Ag.
 28. The method of claim 25, wherein the metal-ammoniacomplex is obtained by reacting a solution comprising a metal salt withone of ammonium hydroxide and ammonia.
 29. The method of claim 25,wherein the reducing agent is a saccharide.
 30. The method of claim 29,wherein the saccharide is an aldose.
 31. The method of claim 30, whereinthe aldose is glucose.
 32. The method of claim 25, wherein thestabilizing agent is a water-soluble resin.
 33. The method of claim 32,wherein the water-soluble resin is a naturally occurring water-solubleresin.
 34. The method of claim 25, wherein the stabilizing agent is oneof a gum arabic and a salt of naphthalene sulphonic-formaldehydeco-polymers.
 35. The method of claim 25, wherein the stabilizing agentis removed during the isolation of the metallic particles.
 36. Themethod of claim 25, wherein the stabilizing agent is removed throughhydrolysis.
 37. The metallic composition of claim 25, wherein theplurality of ultra-fine metallic particles has an average size of lessthan about 100 nm.
 38. The method of claim 25, wherein the plurality ofultra-fine metallic particles has a tight size distribution.
 39. Themethod of claim 38, wherein the plurality of ultra-fine metallicparticles have a tight size distribution when at least about 80% of theplurality of ultra-fine metallic particles has a diameter within a rangeof N±15% N, wherein N is the average diameter of the plurality ofultra-fine metallic particles.
 40. The method of claim 25, wherein theplurality of ultra-fine metallic particles has a high degree ofcrystallinity.
 41. The method of claim 40, wherein at least about 80% ofthe plurality of ultra-fine metallic particles is highly crystalline.42. The method of claim 40, wherein about 100% of the plurality ofultra-fine metallic particles is highly crystalline.
 43. The method ofclaim 25, wherein the plurality of ultra-fine metallic particles has alow degree of agglomeration.
 44. The method of claim 43, wherein thedegree of agglomeration is measured with an I_(agg) value and whereinthe I_(agg1) of the plurality of ultra-fine metallic particles is lessthan about 1.2.
 45. The method of claim 25, at least about 80% of theplurality of ultra-fine metallic particles is not irreversiblyaggregated.
 46. The method of claim 25, wherein the plurality ofultra-fine metallic particles when re-dispersed into a liquid formsdispersion which is stable for at least one week.
 47. The method ofclaim 46, wherein the liquid is water.
 48. The method of claim 47,wherein the dispersion is stable for 12 weeks.
 49. A substrate coatedwith a plurality of ultra-fine metallic particles obtained in accordancewith the method of claim
 25. 50. The substrate of claim 49, wherein aplurality of ultra-fine metallic particles is the particles of a metalselected from the group consisting of Cu, Pd, and Ag.
 51. The substrateof claim 49, wherein the substrate is one of a metallic substrate and anon-metallic substrate.